Normalise decimal WC quantities to integers before sending to Mollie

[mmaymo]

What and why

WooCommerce supports fractional product quantities (e.g. 1.5 metres of fabric) via third-party plugins. When a
cart contained such a quantity, both OrderLines and PaymentLines passed the raw float directly to the Mollie API
as the quantity field. Mollie requires quantity to be a whole number of at least 1, so any order with a
fractional quantity was rejected with a 422 Unprocessable Entity response — making payment completely impossible
for those customers.

How it works now

Both OrderLines and PaymentLines now include a private toIntegerQuantity() helper that converts a fractional
WooCommerce quantity to the smallest equivalent whole number by finding the minimal decimal scale factor (e.g.
1.5 → ×10 → 15, 0.4 → ×10 → 4). get_item_quantity() returns this scaled integer to Mollie, and get_item_price()
divides the line subtotal by the same scaled value — preserving Mollie's required invariant unitPrice × quantity
− discountAmount == totalAmount exactly. For repeating decimals that cannot be scaled exactly within four
decimal places, the helper falls back to rounding and clamps to a minimum of 1.

What to verify in review

Covered by unit tests

• ✓ A fractional quantity of 1.5 is sent to Mollie as integer 15 (scale factor ×10)
• ✓ The unit price for quantity 1.5 is lineSubtotal / 15, keeping unitPrice × 15 − discountAmount == totalAmount
• ✓ A fractional quantity of 0.4 is sent to Mollie as integer 4 (scale factor ×10)
• ✓ A quantity below 0.5 that cannot be scaled exactly (e.g. 1/3) is clamped to 1 rather than 0, preventing
  division by zero
• ✓ An integer quantity such as 3 passes through unchanged
• ✓ OrderLines and PaymentLines produce identical quantity and price values for the same input

Not covered by unit tests

None — all acceptance criteria are covered by the unit test suite.